1. Field of the Invention
The present invention relates to apparatuses for optically reading a target to which optically readable information, such as an information code, for example, a barcode or a two-dimensional code, is attached.
2. Description of the Related Art
Handheld optical information readers aim at reading information codes optically readable, such as barcodes, two-dimensional codes, or other similar codes. In this specification, a target itself or a target to which optically readable information is attached is collectively referred to as the “target”.
For improving the usability of handheld optical information readers, handheld optical information readers that can read an information code positioned at a distance therefrom have been provided.
The handheld optical information reader has a handheld body case provided at its one end portion with a reading window. In addition, the handheld optical information reader also has a photodetector, such as a CCD (Charge-Coupled Device) area sensor, an imaging optics with an imaging lens, and a reading unit composed of a light illuminating device, such as an LED (light emitting diode). The photodetector, the imaging unit, and the reading unit are installed in the body case, respectively.
In order to align the reading window (the photodetector) with the target, handheld optical information readers have been commonly provided with a marker beam irradiating unit using a laser diode (LD), an LED, or other similar light emitting devices. The marker beam irradiating unit is operative to irradiate a marker beam onto the target for indicating a reader's reading position, such as a field of view (FOV) of the photodetector, and/or the center position of the FOV, onto the target.
An example of such marker beam irradiating units is disclosed in U.S. Pat. No. 6,347,163. The disclosed marker beam irradiating unit uses, as a light source, an LD capable of emitting a marker beam with high-visibility, and a diffraction grating through which the emitted beam is irradiated onto the target as various designs thereon.
Specifically, as illustrated in FIG. 15, the marker beam irradiating unit 3 is placed on one side, such as an upper side, of a CCD area sensor 1 and an imaging optics 2. As illustrated in FIG. 16, the marker beam irradiating unit 3 consists of a laser diode 4, a collimation lens 5 disposed at the light emitting side thereof and coaxially aligned therewith, and a diffraction grating 6 disposed at the collimated light output side thereof and coaxially aligned therewith.
In the marker beam irradiating unit 3, a laser beam emitted from the laser diode 4 is collected by the collimation lens 5. The collected beam is transferred therethrough to enter a hologram plane of the diffraction grating 6. The incident laser beam is diffracted by the hologram plane so that the first-order diffraction beamlets and the second-order diffraction beamlets are irradiated from the hologram plane as a marker beam M with a predetermined beam pattern (diffraction pattern) onto a target R (see FIGS. 15 and 17).
For example, the diffraction pattern of the marker beam M consists of four L-shaped pattern elements corresponding to the four corner portions of a field of view V of the CCD area sensor 1 (see FIG. 15). In addition, the diffraction pattern of the marker beam M consists of a cross pattern element indicating the center of the field of view V.
However, because the diffraction beamlets, such as the first-order diffraction beamlets and the second-order diffraction beamlets, form the beam pattern (diffraction pattern) of the marker beam, as illustrated in FIG. 17, the diffraction beamlets are represented on the target R as dot-patterns. That is, the set of dot patterns form a linear pattern on the target R, which may cause the beam pattern on the target R to be smudged, and may deteriorate the visual effects of the beam pattern.
In addition, when scanning a one-dimensional code, such as a barcode, optical information readers with a marker beam irradiating unit can use a marker beam linearly extending in a horizontal direction as illuminating light for scanning the barcode.
Some optical information readers using a marker beam irradiating unit with such a diffraction grating, however, cannot use a marker beam emitted from the marker beam irradiating unit as the illuminating light for scanning the barcode.
Note that the collimate lens 5 has been commonly used as a collective lens for collecting the laser beam emitted from the laser diode 4. Collimate lenses have a function of transferring a laser beam collected thereby with a substantially horizontally prolate ellipsoid profile in its lateral cross section. This may deteriorate the efficiency of transferring the collected beam, causing a greater loss of laser beam.